Gallium nitride (GaN) and its related alloys have been under intense research in recent years due to their promising applications in electronic and optoelectronic devices. Particular examples of potential optoelectronic devices include blue light emitting and laser diodes, and UV photodetectors. Their large bandgap and high electron saturation velocity also make them excellent candidates for applications in high temperature and high-speed power electronics.
Due to the high equilibrium pressure of nitrogen at growth temperatures, it is extremely difficult to obtain GaN bulk crystals. Owing to the lack of feasible bulk growth methods, GaN is commonly deposited epitaxially on substrates such as SiC and sapphire (Al2O3). However, a current problem with the manufacture of GaN thin films is that there is no readily available suitable substrate material which exhibits close lattice matching and close matching of thermal expansion coefficients. Presently, (0001) oriented sapphire is the most frequently used substrate for GaN epitaxial growth due to its low price, availability of large-area wafers with good crystallinity and stability at high temperatures. The lattice mismatch between GaN and sapphire is over 13%. Such huge mismatch in the lattice constants causes poor crystal quality if GaN films were to be grown directly on the sapphire, due to stress formation and a high density of defects, including such defects as microtwins, stacking faults and deep-levels. Typically, these GaN thin films exhibit wide X-ray rocking curve, rough surface morphology, high intrinsic electron concentration and significant yellow luminescence.
Silicon substrates have been considered for use as substrates for growth of GaN films. Silicon substrates for GaN growth is attractive given its low cost, large diameter, high crystal and surface quality, controllable electrical conductivity, and high thermal conductivity. The use of Si wafers promises easy integration of GaN based optoelectronic devices with Si based electronic devices. GaN-based devices have been demonstrated on Si. The direct growth of GaN on substrates such as Si, however, has resulted in substantial diffusion of Si into the GaN film, relatively high dislocation density (˜1010 cm−2) and cracking of the GaN film. GaN is also known to poorly nucleate on Si substrate, leading to an island-like GaN structure and poor surface morphology. Thus, the quality of GaN films grown on silicon has been far inferior to that of films grown on other commonly used substrates such as sapphire or silicon carbide. Moreover, the growth conditions that have been used for GaN on Si are not compatible with standard silicon processes (e.g. the growth temperature is too high).
Numerous different buffer layers have been disclosed for insertion between the substrate and the GaN layer to relieve lattice strain and thus improve GaN crystal quality. ZnO has previously been tested as a buffer layer for Hydride Vapor Phase Epitaxy (HVPE) growth of GaN on sapphire. GaN growth on ZnO/Si structures has also been reported. In general, the use of a ZnO buffer layer produced good quality GaN on both Si and sapphire substrates, even though ZnO is known to be thermally unstable at the high growth temperature of GaN. For ZnO/Si, no continuous two-dimensional GaN layer could be obtained without first growing a low temperature GaN buffer layer to prevent the thermal decomposition of ZnO. HVPE grown GaN films on ZnO/sapphire without this low temperature GaN buffer layer exhibited cracks and peeling when thick (about 200 nm, or more) ZnO buffer layer were grown. It was suggested that the thermal decomposition of ZnO led to the growth of poor quality GaN.